The application of centrifugal pumps is counted today as belonging to the state of the art in almost all technical fields. Typically, centrifugal pumps in the form of centrifugal pump assemblies are applied, consisting of the actual pump and an electrical drive motor mechanically connected thereto.
In order on the one hand to operate the centrifugal pump assembly in an energetic favourable manner, and on the other hand to adapt it as optimally as possible to the application purpose, today, even with small centrifugal pump assemblies of a small construction type, it is counted as belonging to the state of the art to equip these with a speed controller, typically with an electronic frequency converter. Such centrifugal pump assemblies with a speed controller are applied in installations, be it, for example, in heating installations, in sewage installations, in waste water installations, in installations for conveying ground water from a bore hole, to only name a few of typical applications.
It is particularly in installations, but not only there, that it is important, on the one hand to monitor the installation parts, and on the other hand to monitor the process variables. Thus, with centrifugal pump assemblies, it is known to provide a pressure sensor, typically a differential pressure sensor, which detects the pressure between the suction side and the pressure side produced by the pump, thus the delivery head, within the pump housing. Moreover, electrical variables of the motor, such as the power uptake of the motor, and the frequency at which the speed controller feeds the motor, are detected.
However, the detection of the previously mentioned values as a rule is not sufficient for determining the hydraulic operating point, since they permit no information on the delivery rate. The arrangement of flow monitors for detecting the through-flow within the pump is complicated and often prone to malfunction. A flow sensor, with which the flow speed and thus the delivery rate may be detected, is even more complicated and may not be practically applied in waste water technology.
In GB 2 221 073 A, it is counted as belonging to the state of the art, to indirectly compute the delivery rate of the pump, by way of typically determining the filling level of a shaft, in particular the temporal change of the filling level, via a pressure measurement within the shaft. For this, firstly, given a switched-off pump, the average resulting feed quantity per unit of time is determined, and then with the pump switched on, one determines by how much the filling level reduces per unit of time, in order then to conclude the delivery rate under the assumption that the same feed is effected in the time in which the pump runs, as in the time in the pump does not run. This method is complicated since not only is a time measurement additionally required, but also one must also detect the change of the filling level when the pump does not run. Moreover, the accuracy of the detected pump delivery rates depends on the continuity of the feed.